1. Technical Field
The present invention relates to driving circuit for light-emitting diode (LED). More particularly, the present invention relates to a driving circuit that achieves instant LED current shutdown and linear control of the LED.
2. Description of Related Art
The so-called Pulse Width Modulation (hereinafter abbreviated as PWM) refers to a technique for converting analog signals into pulse signals. It primarily serves to monitor the output conditions of a power circuit and to provide signals for controlling electronic components. FIG. 1 shows a current signal converted into a pulse signal with a pulse width of tA in an ideal condition. The waveform of a PWM signal has a leading edge W1 and a trailing edge W2. The leading edge W1 reflects a toggle mode where the PWM signal rises to a high level from a low level, and the trailing edge W2 reflects another toggle mode where the PWM signal descends to the low level from the high level. The time consumed for completing the leading edge W1 is referred to as the rising time t1, and the time consumed for completing the trailing edge W2 is referred to as the falling time t2.
Referring to FIG. 2, it shows a practical PWM waveform of a control circuit. For a practical control circuit of an LED, the falling time t2 related to the trailing edge W2 of a PWM signal indicates the time the LED takes to turn off completely. In other words, when the waveform of the PWM signal is close to the ideal waveform of FIG. 1, the falling time is close to 0, meaning that the LED can be shut down immediately without any time delay. However, referring to the practical PWM waveform shown in FIG. 2, the trailing edge W2 of the PWM signal diverges from the ideal waveform of FIG. 1 so as to lead to undesirable delayed shutdown of the LED.
For instance, assuming the falling time t2 related to the trailing edge W2 of the PWM signal is 0.05 ms, it takes 0.05 ms for the LED to turn off completely. A blinking effect is resulted by delayed shutdown and the blinking effect is favorable only when the time interval i between two blinks is greater than 0.05 ms so that it can be recognized by human eyes. However, for a billboard composed of LEDs and configured to present animations or text scrolls, the delayed shutdown of the LEDs leave shadows around the animated patterns on the billboard and make the animations or text scrolls unrecognizable.
Please refer to FIG. 3, it shows a traditional circuit diagram of a driving circuit for LED. As shown in FIG. 3, the light emission driving circuit 10 serves to drive the LEDs 11. The light emission driving circuit 10 at least includes a PWM unit 12 and a power converting unit 13. For high power LED driver, a switch mode of the power converting unit 13 is used. The power converting unit 13 serves to rectify and regulate an AC power source to predetermined voltage and current values and then generates a driving current signal according to a high-level signal generated by the PWM unit 12. The driving current signal is provided to the LEDs 11 to drive the LEDs 11.
Referring to FIGS. 4a and 4b, they show that the LED current is stopped at time A-E in ideal and practical situations, respectively. In FIG. 4a, when the current is stopped at time A-E, the LED is shut down immediately without any time delay. Therefore, there is a clear difference between PWM width of time B, C and D in the short time. However, the driving current signal output by the power converting unit 13 has a relative slow leading and trailing edge because of the limited switch mode frequency of the power converting unit 13. The power converting unit 13 is used for low power consumption and low-cost driver which has disadvantage of the edge performance. As shown in FIG. 4b, when the LED current is stopped at time B, C or D, there is no difference between PWM width of time B, C and D compared with the ideal situation shown in FIG. 4a. 
Referring to FIG. 5, it shows the relationship between the PWM step and the average LED current in FIG. 3. The brightness of the LEDs 11 is controlled by the current signal input to the LEDs 11. A smooth brightness control is needed so that people will not feel uncomfortable due to the suddenly change of the brightness of LEDs 11. However, in FIG. 5, it is not a smooth line nor shows a linear relationship because of the limited switch mode frequency. Because the current does not vary linearly with the PWM step, the control of the LEDs 11 is not accurate.
Hence, the present invention is herein proposed with the attempt to solve the existing problems mentioned above.